Semiconductor device

ABSTRACT

In a semiconductor device, an AlGaN layer includes a first AlGaN layer and a second AlGaN layer. The second AlGaN layer is positioned between a gate structure portion and a drain electrode and is divided into multiple parts in an arrangement direction in which the gate structure portion and the drain electrode are arranged. A second Al mixed crystal ratio of the second AlGaN layer is less than a first Al mixed crystal ratio of the first AlGaN layer. Accordingly, the semiconductor device is a normally-off-type device and is capable of restricting a decrease of a breakdown voltage and an increase of an on-resistance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage of International PatentApplication No. PCT/JP2016/002872 filed on Jun. 14, 2016 and is based onJapanese Patent Application No. 2015-138957 filed on Jul. 10, 2015, thedisclosures of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a semiconductor device having ahetero-junction structure of nitride semiconductors of gallium nitride(hereinafter, referred to as GaN) and aluminum gallium nitride(hereinafter, referred to as AlGaN).

BACKGROUND ART

Conventionally, in a patent literature 1, a semiconductor device hasbeen proposed that includes a high electron mobility transistor(hereinafter, referred to as HEMT), which is a field-effect transistor,as a horizontal switching device having a hetero-junction structure.

This kind of semiconductor device includes a horizontal HEMT having ahetero-junction structure of GaN and AlGaN. Specifically, thesemiconductor device includes a GaN-type semiconductor layer in which aGaN electron transit layer and an AlGaN electron donor layer are stackedon a substrate in order. The AlGaN electron donor layer has a recess atwhich a thickness is thinned. The semiconductor device includes a gateelectrode in the recess. The semiconductor device includes a sourceelectrode and a drain electrode on the AlGaN electron donor layer andthe gate electrode is sandwiched between the source electrode and thedrain electrode. The recess is provided at a gate implant portion and anouter side of the gate implant portion. The recess includes a firstrecess between the gate implant portion and the source electrode and asecond recess between the gate implant portion and the drain electrode.

In the HEMT, below the AlGaN electron donor layer positioned at bothsides of the gate electrode, carriers of 2-dimensional electron gas(hereinafter, referred to as 2DEG) are induced by piezoelectric effectand intrinsic polarization effect. As such, the HEMT generates a currentbetween the source and the drain through the 2DEG carriers and a channelformed at a surface layer portion of the GaN electron transit layerpositioned below the gate electrode.

In the HEMT, the part of the AlGaN electron donor layer is thinned bythe recess. In the part of the AlGaN electron donor layer having therecess, a stress is eased than the other thicker part. Therefore, in theAlGaN electron donor layer having the recess, an occurrence of thepiezoelectric polarization is restricted and a concentration of the 2DEG(hereinafter, referred to as Ns) is reduced. Accordingly, in the AlGaNelectron donor layer having the recess, decrease of a breakdown voltageis restricted. Also, when the AlGaN electron donor layer is partiallythinned at multiple positions, the maximum electrical field intensity isfurther reduced.

PRIOR ART LITERATURE Patent Literature

Patent literature 1: JP 509399:1 B2

SUMMARY OF INVENTION

However, it has been known that a relation between the thickness and thestress of the AlGaN electron donor layer has a criticality and the Ns ishighly affected by the thickness (see FIG. 3, which will be describedlater). Therefore, there is a possibility that the Ns is largely changedeven when the thickness of the AlGaN electron donor layer is slightlychanged, and it is difficult to control the Ns. The thickness of theAlGaN electron donor layer is thinned by etching and it is difficult tocontrol the thickness and the Ns by the etching.

It is an object of the present disclosure to provide a semiconductordevice being a normally-off type device and being capable of restrictinga decrease of a breakdown voltage and an increase an on-resistance inwell-controlled manner.

According to an aspect of the present disclosure, a semiconductor deviceincludes a horizontal switching device. The horizontal switching deviceincludes a substrate, a channel forming layer, a gate structure portion,a source electrode and a drain electrode. The substrate is formed of asemi-insulator or a semiconductor. The channel forming layer is disposedon the substrate and has a hetero-junction structure of a GaN layer andan AlGaN layer. The GaN layer provides an electron transit layer and theAlGaN layer provides an electron donor layer. The channel forming layerhas a recess at which the AlGaN layer is partially removed. The gatestructure portion has a gate insulation film and a gate electrode. Thegate insulation film is disposed in the recess and the gale electrode isdisposed on the gate insulation film. The source electrode and the drainelectrode are disposed on the channel forming layer and the gatestructure portion is positioned between the source electrode and thedrain electrode. The horizontal switching device generates a currentbetween the source electrode and the drain electrode by a channel formedat a surface layer portion of the GaN layer positioned below a bottom ofthe recess when a 2-dimensional electron gas carrier is induced in theGaN layer adjacent to an interface between the GaN layer and the AlGaNlayer and a voltage is applied to the gate electrode.

In the above configuration, the AlGaN layer includes a first AlGaN layerand a second AlGaN layer. The first AlGaN layer has a first Al mixedcrystal ratio determining a 2-dimensional electron gas concentration ofthe first AlGaN layer. The second AlGaN layer is positioned between thegate structure portion and the drain electrode, and is divided intomultiple parts in an arrangement direction in which the gate structureportion and the drain electrode are arranged. The second AlGaN layer hasa second Al mixed crystal ratio determining a 2-dimensional electron gasconcentration of the second AlGaN layer. The second Al mixed crystalratio is less than the first Al mixed crystal ratio so that the secondAlGaN layer induces a minus fixed charge.

According to the configuration, the semiconductor device is anormally-off-type device and is capable of restricting a decrease of abreakdown voltage and an increase of an on-resistance.

BRIEF DESCRIPTION OF DRAWINGS

The above and other objects, features and advantages of the presentdisclosure will become more apparent from the following detaileddescription made with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a semiconductor device having ahorizontal HEMT according to a first embodiment of the presentdisclosure;

FIG. 2 is a top layout view of the semiconductor device of FIG. 1;

FIG. 3 is a diagram illustrating a relationship between an Al mixedcrystal ratio, a thickness of an AlGaN layer and an Ns;

FIG. 4A is a diagram illustrating energy bands and a carrierconcentration in a cross-section taken along a line IVA-IVA′ of FIG. 1;

FIG. 4B is a diagram illustrating energy bands and a carrierconcentration in a cross-section taken along a line IVB-IVB′ of FIG. 1;

FIG. 5A is a cross-sectional view illustrating a distribution of spacecharges of the horizontal HEMT in a blocking state;

FIG. 5B is a cross-sectional view illustrating a distribution ofelectrons of the horizontal HEMT in an on state;

FIG. 6 is a cross-sectional view illustrating a portion at which anelectrical field intensity is increased when a second AlGaN layer iscontinuously formed;

FIG. 7 is a diagram illustrating distributions of electrical fieldintensities when the second AlGaN layer is divided and when the secondAlGaN layer is continuously formed;

FIG. 8 is a top layout view of a semiconductor device having ahorizontal HEMT according to a second embodiment of the presentdisclosure;

FIG. 9 is a top layout view of a semiconductor device having ahorizontal HEMT according to a third embodiment of the presentdisclosure;

FIG. 10 is a cross-sectional view of a semiconductor device having ahorizontal HEMT according to a fourth embodiment of the presentdisclosure; and

FIG. 11 is a top layout view of a semiconductor device having ahorizontal HEMT according to a fifth embodiment of the presentdisclosure.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present disclosure will be describedwith reference to the drawings. In the following embodiments, parts thatare the same and equal to each other are designed by the same symbols.

(First Embodiment)

A semiconductor device according to the present embodiment will bedescribed with reference to FIG. 1. As shown in FIG. 1, thesemiconductor device according to the present embodiment includes ahorizontal HEMT as a switching device.

The horizontal HEMT of the present embodiment includes a compoundsemiconductor substrate in which i, n, or p-type GaN layer 3 is disposedon a surface of a board 1 through a buffer layer 2. On the surface ofthe GaN layer 3, an AlGaN layer 4 in which a first AlGaN layer 4 a and asecond AlGaN layer 4 b are stacked is formed. The GaN layer 3 and theAlGaN layer 4 provide a hetero-junction structure. The GaN layer 3 andthe AlGaN layer 4 serve as a channel forming layer. The horizontal HEMToperates when a 2DEG carrier is induced in the GaN layer 3 adjacent toan AlGaN/GaN interface by piezoelectric effect and intrinsicpolarization effect.

The board 1 is formed of a semi-insulator material or a semiconductormaterial such as Si(111), SiC or sapphire. The buffer layer 2 is formedon the board 1 to serve as a base film for forming a film of the GaNlayer 3 with good crystallinity. For example, the buffer layer 2 isformed of AlGaN—GaN superlattice layer. When the film of the GaN layer 3is formed on the board 1 with the good crystallinity, the buffer layer 2may be omitted. The above crystallinity means a value that affectselectrical or optical characteristics such as defects or dislocations inthe GaN layer 3.

The GaN layer 3 and the AlGaN layer 4 are formed on the buffer layer 2,for example, by hetero-epitaxial growth.

The GaN layer 3 provides an electron transit layer formed of a GaN-typesemiconductor material such as i-GaN, n-GaN or p-GaN.

The AlGaN layer 4 is formed of a semiconductor material having a bandgapenergy greater than the GaN-type semiconductor material of the GaN layer3. The AlGaN layer 4 provides a barrier layer serving as an electrondonor. In the present embodiment, the AlGaN layer 4 includes the firstAlGaN layer 4 a and the second AlGaN layer 4 b, and a recess 5 in whicha gate structure portion is to be implanted is formed to penetrate thefirst AlGaN layer 4 a and the second AlGaN layer 4 b. The recess 5 isprovided in a line pattern extending in one direction parallel to thesurface of the board 1. Specifically, the recess 5 is provided in a linepattern extending in a vertical direction of a paper of FIG. 2(corresponding to a normal direction of a paper of FIG. 1).

The first AlGaN layer 4 a is formed of Al_(x)Ga_(1−x)N in which xrepresents Al mixed crystal ratio. The second AlGaN layer 4 b is formedof Al_(y)Ga_(1−y)N in which y represents Al mixed crystal ratio. The Almixed crystal ratio x of the first AlGaN layer 4 a is greater than theAl mixed crystal ratio y of the second AlGaN layer 4 b. When the Almixed crystal ratios x and y of the first AlGaN layer 4 a and the secondAlGaN layer 4 b are set as described above, a minus fixed charge isinduced without generating a 2-dimensional hole gas (hereinafter,referred to as 2DHG). Each of the first AlGaN layer 4 a and the secondAlGaN layer 4 b has a thickness in which Ns (which is the 2DEGconcentration) is unambiguously determined by the Al mixed crystalratio.

The thickness of a single layer of the AlGaN layer and the Ns have arelationship shown in FIG. 3. In a region in which the AlGaN layer isthin, the Ns varies largely. In a region in which the AlGaN layer has acertain thickness (which is surrounded by a broken line in FIG. 3), theNs does not depend on the thickness of the AlGaN layer, but the Ns isunambiguously determined by the Al mixed crystal ratio. Accordingly,each of the first AlGaN layer 4 a and the second AlGaN layer 4 b has thethickness in which the Ns does not vary largely depending on thethickness, but the Ns is unambiguously determined by the Al mixedcrystal ratio.

The first AlGaN layer 4 a is formed on the entire upper surface of theboard 1 and is removed in the recess 5. The second AlGaN layer 4 b isformed on an upper surface of the first AlGaN layer 4 a and ispositioned between the gate structure portion and a drain electrode 9,which will be described later. The second AlGaN layer 4 b is dividedinto multiple parts in an arrangement direction in which the gatestructure portion and the drain electrode 9 are arranged. In the presentembodiment, the second AlGaN layer 4 b is positioned with a constantdistance from the recess 5, that is, a side surface of the gatestructure portion. Similarly to the recess 5, each part of the secondAlGaN layer 4 b is provided in the line pattern extending in thevertical direction of the paper of FIG. 3. Therefore, the second AlGaNlayer 4 b is provided in a stripe pattern when viewed from above. Eachpart of the second AlGaN layer 4 b may have an arbitrary width. In thepresent embodiment, each part of the second AlGaN layer 4 b has the samewidth.

In the recess 5, a gate electrode 7 is implanted through a gateinsulation film 6 as the gate structure portion. Specifically, the gateinsulation film 6 having a specific thickness is formed on an inner wallsurface of the recess 5. The gate electrode 7 is formed on the gateinsulation film 6. In this way, the gate structure portion is provided.

For example, the gate insulation film 6 is formed of a silicon oxidefilm (SiO₂) and an alumina (Al₂O₃). The gate electrode 7 is formed of apoly-semiconductor in which a metal such as aluminum, platinum and thelike, or an impurity is doped. The gate insulation film 6 and the gateelectrode 7 are formed in the recess 5, and thereby to provide the gatestructure portion having a MOS structure.

On the other hand, at the both sides of the gate structure portion onthe surface of the first AlGaN layer 4 a, a source electrode 8 and thedrain electrode 9 are formed apart from the second AlGaN layer 4 b. Thesource electrode 8 and the drain electrode 9 are in ohmic-contact withthe first AlGaN layer 4 a. According to the above describedconfiguration, the horizontal HEMT of the present embodiment isprovided.

Although not illustrated, on the surface of the gate electrode 7, thesource electrode 8 and the drain electrode 9, a gate wiring layer, asource wiring layer and a drain wiring layer formed of Al and the likeare formed. The gate wiring layer, the source wiring layer and the drainwiring layer are electrically separated from each other by aninter-layer insulation film and apply arbitral voltage to theelectrodes.

As described above, in the semiconductor device of the presentembodiment, the GaN layer 3 and the AlGaN layer 4 provides thehetero-junction structure, and the AlGaN layer 4 includes the firstAlGaN layer 4 a and the second AlGaN layer 4 b having different Al mixedcrystal ratios. The lower first AlGaN layer 4 a has the Al mixed crystalratio greater than the upper second AlGaN layer 4 b, and the secondAlGaN layer 4 b are divided into multiple parts between the gatestructure portion and the drain electrode 9.

The first AlGaN layer 4 a and the second AlGaN layer 4 b have thethickness in which the Ns is unambiguously determined by the Al mixedcrystal ratio. Energy bands and a carrier concentration of thermalequilibrium state in IVA-IVA′ cross section and IVB-IVB′ cross sectionof FIG. 1 are shown in FIG. 4A and FIG. 4B.

As shown in FIG. 4A, at a portion including only the first AlGaN layer 4a formed on the GaN layer 3, energy bands of a conductive band and avalence band at a boundary between the first AlGaN layer 4 a and the GaNlayer 3 protrudes below. Minus charges gather at the surface layerportion of the GaN layer 3 due to plus fixed charges existing largely inthe first AlGaN layer 4 a being in contact with the GaN layer 3.Accordingly, a carrier concentration, that is, the Ns is increased.

On the other hand, as shown in FIG. 4B, at a portion including the firstAlGaN layer 4 a and the second AlGaN layer 4 b formed on the GaN layer3, minus fixed charges are induced at the surface layer portion of thefirst AlGaN layer 4 a adjacent to an interface between the first AlGaNlayer 4 a and the second AlGaN layer 4 b due to the second AlGaN layer 4b. That is, since the second AlGaN layer 4 b has lower Al mixed crystalratio than the first AlGaN layer 4 a, the minus fixed charges areinduced at the surface layer portion of the first AlGaN layer 4 a.Additionally, the Al mixed crystal ratio of the second AlGaN layer 4 bis set so that 2-dimensional hole gas (hereinafter, referred to as 2DHG)is not generated. Therefore, compared to FIG. 4A, a value subtractingthe minus fixed charges from the plus fixed charges in the first AlGaNlayer 4 a is decreased, and the charge concentration Ns of the minuselectrons gathering at the surface layer portion of the GaN layer 3 isdecreased.

In a blocking state, the fixed charges operate as shown in FIG. 5A.Specifically, at the portion including the second AlGaN layer 4 b on thefirst AlGaN layer 4 a, the minus fixed charges exist at the surfacelayer portion of the first AlGaN layer 4 a and the plus fixed chargespositioned below the first AlGaN layer 4 a are effectively cancelled. Inthis case, an electrical field intensity at a region around the gate isdecreased. On the other hand, at the portion including only the firstAlGaN layer 4 a on the GaN layer 3, an electrical field intensity at aregion around the drain is decreased due to largely existing plus fixedcharges. Accordingly, the semiconductor device does not have the portionin which the electrical field intensity is locally increased, and thebreakdown voltage is improved. Also, since the 2DEG is not generatedbelow the gate structure portion, the semiconductor device is thenormally-off-type device.

In an on state (conduction state), as shown in FIG. 5B, a channel isformed at the surface layer portion of the GaN layer 3 by the 2DEG. Assuch, the current flows between the source and the drain. At the portionincluding the second AlGaN layer 4 b on the first AlGaN layer 4 a,compared to the portion only including the first AlGaN layer 4 a, thenumber of the electrons serving as the carriers (i.e., the number of the2DEG carriers) is decreased. Therefore, when the region including thesecond AlGaN layer 4 b is broad, the on-resistance is increased. In thepresent embodiment, however, the second AlGaN layer 4 b is formed withina predetermined width from the side surface of the recess 5 and thesecond AlGaN layer 4 b is not formed around the source electrode 8 andthe drain electrode 9. Therefore, the increase of the on-resistance isrestricted as far as possible.

As shown in FIG. 6, the semiconductor device may have a structureincluding only one second AlGaN layer 4 b continuously formed betweenthe gate structure portion and the drain electrode 9. In this case,however, as shown by a solid line in FIG. 7, the electrical fieldintensity is increased at a region surrounded by a dashed-dotted line ofFIG. 6, that is, at the end portion of the second AlGaN layer 4 badjacent to the drain electrode 9, in addition to the boundary portionbetween the gate structure portion and the second AlGaN layer 4 b. Inthis case, the electrical field intensity is eased in well-controlledmanner by forming the second AlGaN layer 4 b having the desiredthickness. However, the electrical field intensity is focused at theportion in which the Ns largely varies, such as the end portion of thesecond AlGaN layer 4 b adjacent to the drain electrode 9. Accordingly,the breakdown voltage is not sufficiently improved.

On the other hand, in the present embodiment, the second AlGaN layer 4 bis divided into multiple parts and the Ns largely varies at each part ofthe divided second AlGaN layer 4 b. In this case, as shown by adashed-dotted line of FIG. 7, the electrical field intensity isincreased at multiple portions. Since the electrical field intensity isincreased at the multiple portions, the maximum intensity at eachportion is decreased. Accordingly, the breakdown voltage is furtherimproved compared to the structure including only one second AlGaN layer4 b as shown in FIG. 6.

As discussed above, in the present embodiment, the AlGaN layer 4includes the first AlGaN layer 4 a and the second AlGaN layer 4 b. Thesecond AlGaN layer 4 b is positioned between the gate structure portionand the drain electrode 9 and the second AlGaN layer 4 b is divided intomultiple parts. The first AlGaN layer 4 a has the Al mixed crystal ratiogreater than the Al mixed crystal ratio of the second AlGaN layer 4 b.According to this configuration, the semiconductor device is thenormally-off-type device and the semiconductor device is capable ofrestricting the decrease of the breakdown voltage and the increase ofthe on-resistance.

However, it is preferable to reduce an area of the second AlGaN layer 4b because the on-resistance is increased by the second AlGaN layer 4 b.On the other hand, when the area of the second AlGaN layer 4 b isreduced, the breakdown voltage is affected. Therefore, it is preferableto design the semiconductor device using a design parameter so as toreduce the on-resistance while securing the desired breakdown voltage,the design parameter including a ratio of the area of the portionincluding only the single layer of the first AlGaN layer 4 a and thearea of the portion including the first AlGaN layer 4 a and the secondAlGaN layer 4 b.

A method for manufacturing the above described semiconductor device isbasically similar to the conventional method. However, a step forforming the AlGaN layer 4 is different from the conventional method.

The first AlGaN layer 4 a and the second AlGaN layer 4 b aresequentially stacked on the GaN layer 3 while varying the Al mixedcrystal ratio. Then, the second AlGaN layer 4 b is etched using adesired mask. As such, the second AlGaN layer 4 b is left around aregion in which the gate structure portion is to be formed.

The recess 5 is formed by etching with the mask to penetrate the firstAlGaN layer 4 a from the surface of the second AlGaN layer 4 b and toreach the GaN layer 3.

Then, the gate structure portion is formed by conducting a forming stepof the gate insulation film, an implanting step of the gate electrodeand a patterning step. After that, a forming step of the inter-layerinsulation film and a forming step of the source electrode 8 and thedrain electrode 9 are conducted, and the semiconductor device of thepresent embodiment is manufactured.

In the above described method, the Ns is not adjusted by adjusting thethickness of the first AlGaN layer 4 a and the second AlGaN layer 4 b inthe etching. Therefore, the Ns is not largely varied due to theadjustment of the thickness, and well-controllable and stable devicecharacteristics are expected.

In the above described method, an etch stopper may be employed in theetching of the second AlGaN layer 4 b, for example, by inserting a thinAIN layer not generating the carriers between the first AlGaN layer 4 aand the second AlGaN layer 4 b.

(Second Embodiment)

A second embodiment of the present disclosure will be described. In thepresent embodiment, the configuration of the AlGaN layer 4 is changedfrom the first embodiment. Since the other parts are similar to thefirst embodiment, parts different from the first embodiment will bedescribed.

As shown in FIG. 8, in the second embodiment, a part of the second AlGaNlayer 4 b being in contact with the gate structure portion is providedin the line pattern. A part of the second AlGaN layer 4 b being apartfrom the gate structure portion is not provided in the line pattern butin a dot pattern. Specifically, the second AlGaN layer 4 b is dividedalong a line extending in the horizontal direction of FIG. 8 as well asa line extending in the vertical direction of FIG. 8. The second AlGaNlayer 4 b is provided in a mesh pattern, in other words, in an islandpattern.

When the second AlGaN layer 4 b is arranged in the mesh pattern, thearea of the second AlGaN layer 4 b is reduced compared to the firstembodiment. In the region including the second AlGaN layer 4 b, the Nsis decreased and the on-resistance is increased. Accordingly, in thepresent embodiment, the area of the region having lower Ns is decreasedby providing the second AlGaN layer 4 b in the mesh pattern and therebyto reduce the on-resistance.

(Third Embodiment)

A third embodiment of the present disclosure will be described. Also inthe present embodiment, the configuration of the AlGaN layer 4 ischanged from the first embodiment. Since the other parts are similar tothe first embodiment, parts different from the first embodiment will bedescribed.

As shown in FIG. 9, in the third embodiment, a part of the second AlGaNlayer 4 b being in contact with the gate structure portion is providedin the line pattern. A part of the second AlGaN layer 4 b being apartfrom the gate structure portion is provided in a dot pattern. However,the second AlGaN layer 4 b is not divided along the line extending inthe vertical direction as in the second embodiment, but the second AlGaNlayer 4 b is provided in a staggered pattern.

Accordingly, in the present embodiment, the area of the second AlGaNlayer 4 b provided in the staggered pattern is further decreasedcompared to the second embodiment. Therefore, the on-resistance isfurther decreased.

(Fourth Embodiment)

A fourth embodiment of the present disclosure will be described. Also inthe present embodiment, the configuration of the AlGaN layer 4 ischanged from the first embodiment. Since the other parts are similar tothe first embodiment, parts different from the first embodiment will bedescribed.

As shown in FIG. 10, in the present embodiment, the AlGaN layer 4includes a third AlGaN layer 4 c between the first AlGaN layer 4 a andthe second AlGaN layer 4 b. The third AlGaN layer 4 c is formed ofAl_(z)Ga_(1−z)N in which z represents Al mixed crystal ratio. The Almixed crystal ratio z of the third AlGaN layer 4 c is larger than the Almixed crystal ratio x of the first AlGaN layer 4 a and the Al mixedcrystal ratio y of the second AlGaN layer 4 b. The thickness of thethird AlGaN layer 4 c is thinner than the first AlGaN layer 4 a and thesecond AlGaN layer 4 b.

As described above, in the present embodiment, the AlGaN layer 4includes the third AlGaN layer 4 c between the first AlGaN layer 4 a andthe second AlGaN layer 4 b. An etching rate of the AlGaN layer variesdepending on the Al mixed crystal ratio and the etching rate isdecreased as the Al mixed crystal ratio is increased. Therefore, in themanufacturing process of the semiconductor device, the third AlGaN layer4 c may be used as the etch stopper when the patterning of the secondAlGaN layer 4 b is conducted by the etching. As such, the patterning ofthe second AlGaN layer 4 b is conducted without etching the first AlGaNlayer 4 a more certainly.

(Fifth Embodiment)

A fifth embodiment of the present disclosure will be described. Also inthe present embodiment, the configuration of the AlGaN layer 4 ischanged from the first embodiment. Since the other parts are similar tothe first embodiment, parts different from the first embodiment will bedescribed.

As shown in FIG. 11, the semiconductor device includes an operationregion 100 in which the horizontal HEMT is formed and that operates asan element, that is, an element formed region. The semiconductor devicealso includes an element separation region 200 provided out of theoperation region 100. At an interface between the element separationregion 200 and the operation region 100, the Ns varies largely and theelectrical field is likely to be focused.

Therefore, in the present embodiment, the area of the second AlGaN layer4 b is increased toward the element separation region 200. Specifically,as shown in FIG. 11, the number of the divided parts of the second AlGaNlayer 4 b is increased toward the element separation region 200. At aportion apart from the element separation region 200, that is, at aninside portion of the operation region 100, the number of the dividedparts of the second AlGaN layer 4 b is a constant number smaller thanthe portion near the element separation region 200.

When the area of the second AlGaN layer 4 b is increased toward theelement separation region 200, the variation of the Ns is eased at theinterface between the element separation region 200 and the operationregion 100. Accordingly, the electrical field intensity is decreasedaround the interface and the breakdown voltage is further improved.

(Other Embodiment)

The present disclosure is not limited to the above described embodimentsand may be suitably modified without departing from the gist of thepresent disclosure.

For example, in the embodiments, the recess 5 has the depth exposing thesurface of the GaN layer 3. However, these are just examples. Forexample, the recess 5 may have the depth partially removing the surfacelayer portion of the GaN layer 3. The recess 5 may have the depthleaving the part of the first AlGaN layer 4 a so that the 2DEG carriersare not formed at the bottom of the recess 5.

In the embodiments, the second AlGaN layer 4 b is formed only betweenthe gate structure portion and the drain electrode 9, and the secondAlGaN layer 4 b is not formed between the gate structure portion and thesource electrode 8. However, these are just examples. For example, thesecond AlGaN layer 4 b may be formed with a specific width from the sidesurface of the gate structure portion between the gate structure portionand the source electrode 8.

Furthermore, the layouts of the second AlGaN layer 4 b described in theembodiments are just examples and other layouts may be adopted. Forexample, when the second AlGaN layer 4 b is arranged in the dot pattern,the pattern is not limited to the mesh pattern or the staggered pattern.

Although the present disclosure is described based on the aboveembodiments, the present disclosure is not limited to the embodimentsand the structures. Various changes and modification may be made in thepresent disclosure. Furthermore, various combination and formation, andother combination and formation including one, more than one or lessthan one element may be made in the present disclosure.

The invention claimed is:
 1. A semiconductor device comprising ahorizontal switching device that includes: a substrate formed of asemi-insulator or a semiconductor; a channel forming layer disposed onthe substrate and having a hetero-junction structure of a GaN layer andan AlGaN layer, the GaN layer providing an electron transit layer, theAlGaN layer providing an electron donor layer, and the channel forminglayer having a recess at which the AlGaN layer is partially removed; agate structure portion having a gate insulation film and a gateelectrode, the gate insulation film being disposed in the recess, andthe gate electrode being disposed on the gate insulation film; and asource electrode and a drain electrode disposed on the channel forminglayer, the gate structure portion being positioned between the sourceelectrode and the drain electrode, the horizontal switching devicegenerating a current between the source electrode and the drainelectrode by a channel formed at a surface layer portion of the GaNlayer positioned below a bottom of the recess when a 2-dimensionalelectron gas carrier is induced in the GaN layer adjacent to aninterface between the GaN layer and the AlGaN layer and a voltage isapplied to the gate electrode, wherein the AlGaN layer includes a firstAlGaN layer and a second AlGaN layer, the first AlGaN layer has a firstAl mixed crystal ratio determining a 2-dimensional electron gasconcentration of the first AlGaN layer, the second AlGaN layer ispositioned between the gate structure portion and the drain electrode,the second AlGaN layer is divided into multiple parts in an arrangementdirection in which the gate structure portion and the drain electrodeare arranged, the second AlGaN layer has a second Al mixed crystal ratiodetermining a 2-dimensional electron gas concentration of the secondAlGaN layer, and the second Al mixed crystal ratio is less than thefirst Al mixed crystal ratio so that the second AlGaN layer induces aminus fixed charge.
 2. The semiconductor device according to claim 1,wherein the gate structure portion, the source electrode and the drainelectrode are provided in a line pattern extending in one directionparallel to a surface of the substrate, and the second AlGaN layer isprovided in the line pattern extending in the one direction.
 3. Thesemiconductor device according to claim 1, wherein the gate structureportion, the source electrode and the drain electrode are provided in aline pattern extending in one direction parallel to a surface of thesubstrate, and the second AlGaN layer is provided in a dot pattern. 4.The semiconductor device according to claim 3, wherein the second AlGaNlayer is provided in the dot pattern in which the second AlGaN layer isdivided into multiple parts along a line extending in the one direction.5. The semiconductor device according to claim 3, wherein the secondAlGaN layer is provided in the dot pattern in which the second AlGaNlayer is arranged in a staggered pattern.
 6. The semiconductor deviceaccording to claim 1, wherein the AlGaN layer includes a third AlGaNlayer positioned between the first AlGaN layer and the second AlGaNlayer, and the third AlGaN layer has a third Al mixed crystal ratio thatis greater than the first Al mixed crystal ratio of the first AlGaNlayer and greater than the second Al mixed crystal ratio of the secondAlGaN layer.
 7. The semiconductor device according to claim 1,comprising: an operation region in which the horizontal switching deviceis provided and that operates as an element; and an element separationregion that is positioned out of the operation region, wherein an areaof the second AlGaN layer is increased toward the element separationregion.
 8. The semiconductor device according to claim 1, wherein the2-dimensional electron gas concentration of the first AlGaN layer isincreased as the first Al mixed crystal ratio is increased, and the2-dimensional electron gas concentration of the second AlGaN layer isincreased as the second Al mixed crystal ratio is increased.
 9. Thesemiconductor device according claim 1, wherein the second AlGaN layeris divided into the multiple parts in the arrangement direction of thegate structure portion and the drain electrode, and the multiple partsare apart from each other.
 10. The semiconductor device according toclaim 1, wherein the first AlGaN layer is disposed on the GaN layer, andthe second AlGaN layer is disposed on the first AlGaN layer.